In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities, when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which incorporates one or more diverse kinds of filler materials such as particulates, fibers, rods, and the like.
There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. Several copending patent applications and one Patent, all assigned to the same owner as this application (hereinafter referred to as Commonly Owned Patent Applications and Patent), overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. The method is disclosed generically in commonly owned U.S. Pat. No. 4,713,360, which issued on Dec. 15, 1987 from U.S. application Ser. No. 818,943, filed Jan. 15, 1986, which is a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985, which is a continuation-in-part of U.S. application Ser. No. 591,392, filed Mar. 16, 1984, all in the names of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods for Making the Same". These applications disclose the method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a parent metal. Molten metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation reaction product toward the oxidant thereby continuously developing a ceramic polycrystalline body which can be produced having an interconnected metallic component. The process may be enhanced by the use of an alloyed dopant, such as is used in the case of oxidizing aluminum doped with magnesium and silicon in air to form alpha-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the precursor metal, as described in commonly owned U.S. Pat. No. 4,853,352, which issued Aug. 1, 1989, from U.S. application Ser. No. 220,935, filed June 23, 1988, and entitled "Method of Making Self-Supporting Ceramic Materials and Materials made Thereby", in the names of Newkirk et al., which was a continuation application of U.S. patent application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, which is a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, all in the names of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials".
This oxidation phenomenon was utilized in producing ceramic composite bodies as described in commonly owned U.S. Pat. No. 4,851,375, which issued July 25, 1989, as "Methods of Making Composite Ceramic Articles Having Embedded Filler", in the names of Marc S. Newkirk et al., which issued from U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, both in the names of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same". This Patent discloses novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in the commonly owned and copending U.S. patent application Ser. No. 338,471, filed Apr. 14, 1989, which is a continuation application of U.S. patent application Ser. No. 861,025, filed May 8, 1986, both in the names of Marc S. Newkirk et al and entitled "Shaped Ceramic Composites and Methods of Making the Same". In accordance with the method in these U.S. patent applications, the developing oxidation reaction product infiltrates a permeable preform in the direction towards a defined surface boundary. It was discovered that high fidelity is more readily achieved by providing the preform with a barrier means, as disclosed in commonly owned and copending U.S. patent application Ser. No. 861,024, now allowed, filed May 8, 1986, in the names of Marc S. Newkirk et al and entitled "Method of Making Shaped Ceramic Composites With the Use of a Barrier". This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a parent metal to a barrier means spaced from the metal for establishing a boundary or surface. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern is disclosed in commonly owned U.S. Pat. No. 4,828,785, which issued on May 9, 1989, from U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986, in the names of Marc S. Newkirk et al and entitled "Inverse Shape Replication Method of Making Ceramic Article and Articles Obtained Thereby", and in U.S. Pat. No. 4,859,640, which issued Aug. 22, 1989, from U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986 in the name of Marc S. Newkirk and entitled "Method of Making Ceramic Composite Articles With Shape Replicated Surfaces and Articles Obtained Thereby".
The aforementioned Commonly Owned Patent Applications and Patents, incorporated herein by reference, disclose methods for producing ceramic articles which overcome some of the traditional limitations or difficulties in producing ceramic articles as substitutes for metals in end-use applications.
Common to each of these Commonly Owned Patent Applications and Patents is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product interconnected in one or more dimensions (usually in three dimensions) and one or more metallic constituents or components. The volume of metal, which typically includes non-oxidized constituents of the parent metal and/or metal reduced from an oxidant, dopant or filler, depends on such factors as the temperature at which the oxidation reaction product is formed, the length of time at which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, the presence of reduced constituents of any oxidant or filler materials, etc. Although some of the metallic components may be isolated or enclosed, it is frequently the case that a substantial volume percent of metal will be interconnected and accessible from an external surface of the ceramic body. It has been observed for these ceramic bodies that this interconnected metal-containing component or constituent can range from about 1 to about 40 percent by volume, and sometimes higher. Such a metallic component can impart certain favorable properties to, or improve the performance of, the ceramic articles in many product applications. For example, the presence of metal in the ceramic structure may have a substantial benefit with respect to imparting fracture toughness, thermal conductivity, or electrical conductivity to the ceramic body.
The present invention discloses a method for fabricating a ceramic or ceramic composite body, with the addition of a process modifier, exhibiting a microstructural alteration as compared to substantially the same ceramic or ceramic composite body fabricated without a modifier. An altered microstructure may improve or contribute to one or more properties of the resulting ceramic product.
The entire disclosures of all of the foregoing Commonly Owned Patent Applications and Patents are expressly incorporated herein by reference.